1. Field of the Invention
The invention relates to the art of making silicon compounds and, more particularly, to making ultrapure silicon imides or amides useful as precursors for making silicon nitride.
2. Description of the Prior Art
Silicon nitride does not occur in natural deposits in processible quantities. It must be chemically formed and processed. The principal methods by which silicon nitride has been made heretofore, include: (1) reacting nitrogen gas with silicon powder at a high temperature, but which temperature is below the melting point of silicon (see U.S. Pat. No. 3,839,541); (2) reduction of silica coupled with gas nitriding at high temperatures (see German Pat. No. 88999 to Mehner); and (3) vapor phase reaction of a silicon halide (i.e., SiCl.sub.4) and ammonia in a gas-to-gas reaction at 1100.degree.-1350.degree. C. (see U.S. Pat. No. 4,145,224); and (4) formation of silicon imide or amide and subsequent thermal decomposition of such imide or amide. Silicon imide has the formula Si(NH).sub.2, whereas silicon amide has the formula Si(NH.sub.2).sub.4.
Silicon amide or imide has been formed by any one of the following: (a) a liquid-to-gas reaction of silicon halide and ammonia in the presence of benzene or hexane at -10.degree. C. to 5.degree. C. for 0.5-2 hours (see U.S. Pat. No. 3,959,446); (b) a solid-to-gas reaction of silicon halide and ammonia at about -196.degree. C. (see O. Glemser and P. Naumann, "Uber den thermischen Abbau von Siliciumdiimid Si(NH).sub.2 ", Zeitschrift fur Anorganiche und Allgemeine Chemie, Band 298, 134-141 (1959); and (c) a liquid-to-liquid organic boundary phase reaction of silicon halide and ammonia at significantly subzero temperatures (see U.S. Pat. No. 4,196,178).
Each of the known methods for making imide or amide precursors suffers from certain deficiencies. Formation of a silicon amide or imide by a liquid-to-gas reaction results in poor purity of the resulting silicon nitride powder since a great deal of residual halides and carbon is present.
Forming silicon amide or imide by reacting a gas with a solid is disadvantageous because the exothermic reaction destabilizes the temperature of the ingredients, causing the solid ammonia to thaw. In the Glemser article, silicon chloride is transported under vacuum as a vapor to react with solid NH.sub.3 at about -196.degree. C. When the solid ammonia thaws, there is no means to control the reaction and, therefore, the process must be stopped to allow for the transport of the reaction mix to another station to eliminate the byproducts of the reaction, preferably through filtering. The reaction flask is warmed and is vigorously shaken. The reaction mix is not in intimate contact with any filter. This causes the entire processing to be accomplished in five to seven distinct stages or portions; the risk of plugging and contaminating the filter frit remains and can only be obviated by boiling the NH.sub.3, changing the cooling bath to -80.degree. C., and then cooling the second flask to -20.degree. C. This cycling between -196.degree. C. to -80.degree. C. for the cooling bath and -80.degree. C. to -20.degree. C. for the second flask is repeated 10-12 times before continuing to add silicon chloride. This is an unusually complex and troublesome process procedure; it does not lead to a particularly purified product because of the transporting of byproducts between stages and through contaminating hoses and channels.
Forming silicon imides or amides by a liquid-to-liquid reaction holds a great deal of promise for providing purity in the resultant silicon nitride powder, but known techniques have found the reaction unusually violent and uncontrollable, as evidenced in U.S. Pat. No. 4,196,178 Ube. At column 1, lines 66-68 and column 2, lines 1-10, the reaction is indicated as being vigorously exothermic and evolves ammonium halide which will be given off as fumes unless contained. These fumes create uncontrolled deposition and plugging problems for the processing apparatus. If nothing further is done to the process other than to bring together such liquids at super-depressed temperatures, the uncontrollability of the reaction makes such process not only uneconomical but unfeasible for commercial use. The Ube patent attempts to provide some control to such reaction by incorporating an organic solvent immiscible in the liquid ammonia, thereby forming a interfacial surface through which the silicon halide slowly diffuses and only a small portion of the silicon halide additive reaches the liquid ammonia at any one specific moment. Although the violent and vigorous reaction is controlled somewhat, this is done at the extreme disadvantage of contaminating the resultant product with carbon carried by the organic solvent. In fact, the silicon halide is mixed with the organic solvent for purposes of transport into the reaction vessel. Not only is the change in the technical process not worth the disadvantage of severe carbon contamination, but the process is made uneconomical because it requires the added expense of organic solvent materials and handling costs, and the process must be carried out in interrupted batches since the chemical product cannot be immediately and simultaneously washed away with liquid ammonia due to the presence of the organic solvent. To carry out any washing and elimination of solid precipitates, other than the silicon imide, requires that the entire process be stopped and transferred to another vessel for cleansing.
The inventors in U.S. Pat. No. 4,196,178 mistakenly believed their powder was pure because they did not test for the presence of carbon. The off-white color of their calcined imide is due to the presence of carbon. Chemical analysis by high temperature combustion infrared analysis of the powder prepared by the method of U.S. Pat. No. 4,196,178 showed a presence of 1700 ppm carbon.
It is an object of this invention to provide a method of making an ultrapure silicon nitride precursor. A particular object of this invention is to form silicon imide or amide by a liquid-to-liquid reaction with greater controlability and without contamination.